1. Field of the Invention
The present invention relates to a process for producing 2,6-naphthalenedicarboxylic acid, and more particularly to a process for producing 2,6-naphthalenedicarboxylic acid industrially advantageously by liquid phase oxidation of 2,6-dimethylnaphthalene, wherein a mother liquor which is obtained after separation of crystals of 2,6-naphthalenedicarboxylic acid from a product of the liquid phase oxidation (referred to as a mother liquor of oxidation, hereinafter) is efficiently recycled to the reaction system, and the oxidation catalyst is reused.
2. Description of the Related Arts
2,6-Naphthalenedicarboxylic acid and esters thereof are useful as raw materials for high performance polyesters.
2,6-Naphthalenedicarboxylic acid is produced by oxidation of a 2,6-dialkylnaphthalene in a solvent containing a lower aliphatic carboxylic acid in the presence of a catalyst containing cobalt, manganese, and bromine (U.S. Pat. No. 3,856,855).
In this oxidation, the naphthalene ring itself tends to be oxidized to form trimellitic acid. Trimellitic acid thus formed tends to form strong complexes with the metal catalyst for oxidation, and it is difficult to separate and remove the metals in the complexes from 2,6-naphthalenedicarboxylic acid. A mother liquor of oxidation obtained after separation of crystals of 2,6-naphthalenedicarboxylic acid from the reaction product contains, as described above, trimellitic acid along with the useful metal catalyst for oxidation. Generally in the production of an aromatic carboxylic acid by the liquid phase oxidation of an aromatic hydrocarbon, relatively expensive catalyst components, such as cobalt, manganese, and bromine, are used, and it is economically desirable that the catalyst components are recycled and reused (Japanese Patent Application Laid-Open No. Heisei 4(1992)-266846). However, when the mother liquor of oxidation is recycled into the oxidation system as described above, a problem arises in that trimellitic acid forms complexes with the metal catalyst for oxidation to deactivate the catalyst.
To solve the above problem that the metal catalyst for oxidation is deactivated by trimellitic acid, a process is described in Japanese Patent Application Laid-Open No. Heisei 7(1995)-48314 in which, when 2,6-dimethylnaphthalene is oxidized with a gas containing molecular oxygen in a solution in which a catalyst containing heavy metals and bromine is dissolved in a solvent containing a lower aliphatic carboxylic acid, the catalyst is recycled and reused after the catalyst is supplemented so that the concentration of the heavy metals of the catalyst, excluding the heavy metals in the form of complexes with ortho-benzenedicarboxylic acids, in the solvent is 0.2% by weight or more.
However, the increase in the yield is not sufficient although the catalyst is supplemented so that the concentration of the heavy metals of the catalyst, excluding the heavy metals in the form of complexes with ortho-benzenedicarboxylic acids, in the solvent is 0.2% by weight or more.
As described above, a larger recycled fraction of the mother liquor of oxidation is preferable because, by recycling and reusing the mother liquor of oxidation, the catalyst components and the solvent components in the mother liquor of oxidation can be utilized efficiently and the raw materials, intermediate products, and fine particles of 2,6-naphthalenedicarboxylic acid contained in the mother liquor of oxidation can be recovered. However, the above mother liquor of oxidation contains small amounts of impurities and byproducts in addition to the useful components described above, and substances adversely affecting the oxidation are accumulated by recycling and reusing the mother liquor of oxidation to decrease the yield.
Unlike the commercially established processes for producing terephthalic acid, the process for producing 2,6-naphthalenedicarboxylic acid has a problem in recycling and reusing the mother liquor of oxidation as the solvent for the oxidation in that particles of crystals of 2,6-naphthalenedicarboxylic acid formed in the step of oxidation are very small, and increasing the particle size of the crystals is difficult even by treatments in a plurality of crystallization tanks because solubility of 2,6-naphthalenedicarboxylic acid is very small in the solvent for the reaction. Therefore, it is very difficult that complete separation in the step of solid-liquid separation can be achieved.
As the result, the mother liquor of oxidation contains a portion of crystals of 2,6-naphthalenedicarboxylic acid, particularly crystals having smaller sizes than the average size of crystals grown in the step of oxidation. When the mother liquor of oxidation containing very fine particles is recycled into the reactor for the oxidation and repeatedly used as the solvent for the oxidation, the content of fine crystals in a slurry supplied to the step of solid-liquid separation is increased, and the desired ability of solid-liquid separation cannot be exhibited.